This invention relates generally to a system and method of initiating and relaying messages, and more particularly to a two-way paging system using portable pagers (tags) that receive paging signals and initiate or transmit reply messages using a cellular communications network.
The present invention is used in a communication region and particularly for communicating messages in a communication region. The present invention is particularly useful where large numbers of pagers are present in the communication region, where the locations or identities of the pagers in the communication region are not necessarily known, where transport of the pagers to and from the communication region is not necessarily restricted and where contentions among communications to and from pagers need to be resolved in a time and energy efficient manner.
Conventional radio paging systems are limited in that a person having a pager can only receive messages that are sent to the pager. Typically, a person desiring to send a page dials a phone number reserved for the pager. A central paging system answers the call and decodes tones that are subsequently entered by the paging party. Usually, the tones entered by the paging party represent a telephone number the paging party would like one recipient to call. onus, One message received by the portable pager generally calls for some response by the recipient, such as calling the telephone number displayed on the display of the pager. The recipient must normally find some conventional method to contact the party that initiated the page. Typically, the recipient calls the paging party on a telephone to respond.
Conventional paging systems suffer from a number of shortcomings. For instance, there are times when the recipient of a page may not have a telephone available from which to call the paging party. In addition, the paging party typically does not know whether the page was received by the paged party, which is particularly troublesome where the page is urgent. Furthermore, the user of a portable pager may want to initiate transmission of a message regardless of whether a page has been received. Yet another shortcoming is that there is typically no way to track the location of portable pagers with conventional paging systems.
It would be desirable to have a simple method of sending or receiving messages via portable pagers (pagers) independent of a telephone network, as well as determining the location of persons or items associated with those pagers.
The communication region in which the communication occurs may be small or large, cellular or single celled or may have other characteristics. Simple communication procedures may be adequate for small numbers of pagers (pagers), however, for large numbers of pagers, the simple procedures become complex and time consuming. For large numbers, the methods that may work for small numbers are inadequate.
Thus a need exists for an accurate and efficient system that communicates with large numbers of pagers. The system must operate in a time and energy efficient manner.
A number of communication systems exist and these systems are based upon many different technologies. Some of the communications may be in a broadcast mode (one to many) where an interrogator broadcasts to many pagers, others may be in a one-to-one mode where communication is between one interrogator and one pager.
To increase the power of incident radiation and thereby increase the range of a passive reflector system, passive reflector systems have employed focused radiation rather than omni-directional radiation since the incident power of focused radiation tends to be greater than the incident power of omni-directional radiation. Focused radiation, however, is not practical for a location system because it requires prior knowledge of the location and direction of the pager with respect to the transmitting source. Although reflective systems are used as verification or security systems, reflective systems have not proved practical for identification systems for items of unknown location or in an unstructured environment.
Another example is a communication system for interrogating transient pagers brought into the field of an interrogator where the interrogator sends a synchronization signal to responsive pagers and identifies the responding pagers with no acknowledgement to a pager to communicate to the pager that a successful transmission was received by the interrogator. In that system, the interrogator continuously broadcasts interrogation requests and listens for and records, when able, pagers which respond. The collisions which inevitably result from two or more pagers responding simultaneously to the interrogator are attempted to be overcome by having the pagers indefinitely repeat their transmissions at randomly chosen times. Such a system tends to create an unacceptable collision problem in the case of many pagers or stationary pagers and hence is limited to identification of only a few pagers and then only if the few pagers are transient at the interrogator station.
As another example, a communication system uses two frequencies, one for interrogators to send and the other for pagers to respond using various communication sequences. If more than one pager responds, the pager signals collide and the interrogator win detect errors and copy those errors back to the pagers. The pagers transmit again and frequently again collide repeating the error transmissions. The pagers then go silent and respond again after a random time delay. Such a system is deficient in organizing the energy resource. The system is limited to only a few pagers since if expanded to a large number of pagers, the system presents an unacceptable level of energy consumption due to the disorganized method of resolving collisions. The response acknowledge cycle of each single pager, along with the associated time overhead in error determination, requires constant transmission of signals, consuming an excessive amount of power.
Still another communication system employs multiple frequency responses to interrogation where a transmitter transmits messages to a group of pagers. The transmitter transmits pager addresses in a time-multiplexed group on a single frequency to normally sleeping pagers. All pagers wake up and listen to the address to determine if their unique address is contained in the group and if so, at what relative position in the sequence. If a pager determines that its address is not in the group of addresses sent, the pager returns to sleep. If the pager determines that its address is in the group, it remains awake to receive a message sent by the central transmitter. Having received its message, the pager sends back to the central transmitter a response signal on a frequency specified by the relative position of the pager address in the group address transmission. Such a system limits the number of pagers that can respond at any one time to the number of frequencies available for responses. Only a small number of frequencies, perhaps as few as twenty, are practical due to design tradeoffs between the number of frequencies needed and the frequency precision required of the transmitter and receiver design. The antenna design also becomes more expensive and complex due to the wide frequency bandwidth within which such systems must be responsive.
The communication systems described by way of example, and other proposed systems, are unable to satisfy the need to identify one or more of a plurality of pagers within a given area, to resolve collisions in the responses of interrogated pagers and to accomplish these tasks in a time and energy efficient manner for a large number of pagers.
In view of the above background, there is a need for highly efficient communication systems capable of operation in an orderly and time and energy efficient manner with large numbers of pagers to communicate with all pagers.
For an effective communication system for communicating with items in a communication region, many factors must be considered including the following.
The size of the communication region determined in part by the communication range of the signals from interrogator to pagers and from pagers to interrogator.
The rate at which pagers are introduced into and removed from the communication region.
The number of pagers which are within the communication region at any one time where a large number may be hundreds or thousands or more and a small number may be none or a few.
The nature and number of communication channels between the pagers and the interrogators.
The bandwidth of the communication channels between the pagers and the interrogators.
The reliability of the communication channels.
The efficiency of time with which the interrogation process can be completed and the speed of communications.
The type of communication protocol that is employed.
The cost of the system and particularly the cost of each pager.
Power requirements including battery life and size for portable operation.
Additional desirable features of an identification system are the ability to increase the range of the system over a larger communication region by forming adjacent communication cells in a cellular system where each cell includes an interrogator that communicates with pagers over a part of the larger communication region so that a plurality of such interrogators together effectively communicate over the entire communication region. Such a system, having coordinated communications among the cells, defines a wide area system.
In summary, efficient communication systems are needed that communicate with, pagers within a communication region. Since the number of pagers may be hundreds or thousands, the communication protocol is significant and must consider cost, reliability, accuracy, energy efficiency and the other factors identified above. Also, since pagers are transportable when attached to transportable items, the pagers are typically battery operated and hence the need to conserve power in order to extend battery life is of major consideration.